HDD Challenge: Pipeline River and Levee Crossing Combines Trenchless and Over-the-Top Construction Methods
March 30, 2016How can you install a pipeline under the Atchafalaya River with trenchless construction while still providing the most protection to the adjacent US Army Corps of Engineers’ (USACE) flood protection levee?
GeoEngineers Inc. saw that the circumstances of the Bayou Jack Extension Pipeline project in St. Landry and Point Coupee Parishes, Louisiana could make this happen, and the thoughtful project engineering ended up saving owner, Crosstex Energy (now EnLink Midstream), time and money in the process.
Crosstex needed to install 30.4 miles of new 16-in. diameter steel gas pipeline across the Atchafalaya River and the USACE-regulated East and West Atchafalaya Protection Levees, as well as the Upper and Lower Morganza Spillway Guide Levees to connect its existing Bayou Jack Lateral with the False River Pipeline. As part of this Bayou Jack Extension Pipeline project, Crosstex first planned to complete a long (around 5,000 ft) horizontal directional drill (HDD) under the East and West Protection levees and Atchafalaya River, as well as two additional HDDs under the Morganza Spillway levees. Crosstex asked GeoEngineers to help with the levee permitting for the pipeline crossings.
After looking at the project site, GeoEngineers suggested going over the top of the levees with conventionally laid construction, which requires less permitting and review time due to substantially less construction risk to the levee foundation soils, and installing an HDD only under the river. The project site had room in the batture, or the alluvial land between a river at low-water stage and a levee, for the entry and exit points of the HDD.
By not subdrilling the pipeline under the levees, the HDD length was cut in half to a design length of 2,205 ft, thereby cutting construction risk, expenses and also permitting time. In the past, permitting for pipelines going under USACE-regulated levees could take three to six months. Starting in 2015, permitting time has increased since the USACE has implemented national review of levee subdrill permits through its Risk Management Center — all in an effort to keep the public safe through closely monitored levee integrity.
With the USACE relieved with the pipeline going over the levees instead of under them, and Crosstex pleased with a faster and less expensive pipeline installation, GeoEngineers proceeded with the geotechnical and HDD design engineering of the project.
Miller Engineers & Associates Inc. (Miller) of Franklin, La., performed boundary, topographic and hydrographic/bathymetric surveys to gather all land and water based data required for the routing and design of the pipeline. Information was gathered using a combination of GPS and total station surveys in terrain that varied from sugar cane fields to bottomland hardwoods and cypress swamp. Miller also generated alignment sheets, property plats, USACE permit drawings and participated in construction staking and preparation of as-built alignment sheets.
Geotechnical Engineering and Levee Crossing Design
GeoEngineers explored subsurface conditions at the over-the-top levee crossing sites and near the HDD alignment by drilling six geotechnical soil borings to depths up to 180 ft below ground surface (bgs) and advancing three cone penetration tests (CPT) to depths up to 200 ft bgs adjacent to the proposed alignment. After field work concluded, a comprehensive laboratory testing program was completed. GeoEngineers strength tested more than 70 percent of the soil samples obtained in the field and visually classified 100 percent of the soil samples obtained in the field within their onsite laboratory at their Baton Rouge, La., Office.
GeoEngineers used the soil parameters obtained from its subsurface investigation to analyze the influence of the proposed over-the-top pipeline on the levee and evaluate the HDD alignment.
For the levee crossing engineering, GeoEngineers used the USACE’s DOS-based, 2-dimensional stability program, Method of Planes (MOP), to show the conventionally laid, over-the-top crossings and HDD alignment met the USACE’s required factors of safety for levees. These engineering calculations included slope stability and expected settlement of the levees both before and after pipeline installation for low and high water conditions and the determination of a stability control line for each levee. After the soil behavior was evaluated, GeoEngineers designed the over-the-top pipeline levee crossings.
HDD Design Engineering
GeoEngineers evaluated site survey data, reviewed the subsurface soil data near the vicinity of the proposed HDD alignment, completed a laboratory testing program and prepared a detailed HDD design for the project. Because the entry and exit workspaces were located inside the levees, flooding during high water events had to be considered. Construction activities took place in April and May 2014.
The exit point for the HDD was located approximately 700 ft west of Louisiana State Route 417. The temporary workspace measured 200 ft wide by 200 ft long and was in a field that didn’t require site preparation before trenchless construction.
The pipe stringing and fabrication area was located east of Louisiana State Route 417 within an agricultural field, and the area had to sufficiently allow for the fabrication and stringing of one continuous section of pipe. During pullback, the product pipe passed through the agriculture field, over State Route 417, and over the East Protection Levee, which meant the contractor, Ranger Directional Inc., had to make a suitable support system for handling and supporting the product pipe during pullback operations. WHC Inc. completed the mainline pipeline work including the over-the-top levee crossings.
The entry point for the HDD was located approximately 450 ft east of Louisiana State Route 105 that is adjacent to the West Protection Levee and approximately 275 ft west of the Atchafalaya River. The temporary workspace area required significant clearing and tree removal for the project.
Since both the HDD entry and exit points were located within the batture of the levees, temporary access ramps over the levees were required to access workspace while still protecting the levees. GeoEngineers’ engineering design also had to include calculating the levee’s stability control line — which is a theoretical limit of how much the river could scour toward the levee and still maintain the USACE required factor of safety against global stability failure — and fit the HDD outside of this line.
To complete the entire project, the HDD was constructed first under the Atchafalaya River with the pipeline going over the top of the Atchafalaya River East and West Protection Levees, as well as the Morganza Spillway Lower and Upper Guide Levees after the HDD was successfully completed.
The HDD construction took approximately three weeks and went smoothly. The crossing was completed without incident and did not cause any delays to the overall project.
From receiving the USACE’s Letter of No Objection to complete work within 1,500 ft of the levees on March 27, 2014, the over-the-top levee pipeline crossings (East and West Atchafalaya Levees and the Upper and Lower Morganza Spillway Levees) and the Atchafalaya River HDD were completed and in-service in approximately two months (by the end of May).
Since Crosstex wasn’t drilling under the levee, there was reduced risk and correspondingly less regulatory direction of the project. Today, EnLink’s river crossing designs follow the lessons learned from this project.
Considering trenchless design engineering alternatives early during project routing can offer considerable savings of time and money for pipeline owners. The combination of over-the-top levee crossings with an HDD installation from batture to batture under the river allows for a shorter HDD length, the ability to use smaller equipment in construction, and less permitting and review due to the lesser risk to levee foundation soils. Pipeline owners should consider the shortest or most direct route might not always be the easiest way to cross an obstacle. Looking upstream and downstream for ample space between the river and levee for HDD entry and exit points, and routes could be adapted to accommodate these types of crossings where appropriate.
James Vincent, P.E., is a civil engineer with GeoEngineers Inc.